The present invention relates to the field of mobile radio communication systems.
Prior art mobile radio communication systems may be divided up into cells. Mobile telephones or mobile radios, henceforth referred to as mobile stations, can move within a cell. Mobile stations can also move from one cell to another.
FIG. 1 illustrates a single cell of a cellular radio system. The elements shown in FIG. 1 show the general scheme of a personal mobile radio (PMR) system 10. Portable radios 2, 4 and 6 of FIG. 1 can communicate with a base station 8. Radios 2, 4 and 6 could equally well be mobile radios mounted in vehicles. Each of the radios shown in FIG. 1 can communicate through base station 8 with one or more other radios. If radios 2, 4 and 6 are capable of direct mode operation, then they may communicate directly with one another or with other radios, without the communication link passing through base station 8. Radios 2, 4 and 6 constitute mobile stations in this system.
Examples of prior art radio systems of the general type illustrated in FIG. 1 are the iDEN and TETRA radio systems.
FIG. 2 shows the arrangement of several cells 110, 120, 130, 140, 150 of a cellular radio system. A xe2x80x98Software and Measurement Infrastructurexe2x80x99 (SwMI) 100 controls these five cells. The SwMI and the cells which the SwMI controls together constitute a radio xe2x80x98networkxe2x80x99. The SwMI is responsible for a variety of functions. These typically include control of the assignment of IP addresses to mobile stations operating in the network, and the routing of communications to and amongst mobile agents in the network. A radio such as those shown as 2, 4 or 6 in FIG. 1 can move from cell to cell within the network, communicating in any of the cells.
The SwMI 100 shown in FIG. 2 performs several functions, indicated as xe2x80x98FAxe2x80x99, xe2x80x98IPxe2x80x99 and xe2x80x98HAxe2x80x99 in block 100. The xe2x80x98IPxe2x80x99 box indicates that the SwMI is responsible for allocating IP addresses (numbers) to radios operating within the cells 110-150. The IP address allows packets of data with the correct IP address to be delivered to the radio within the network. The SwMI will keep a table showing the cell within which a mobile radio is located, in order to be able to route calls to a base station located within communication range of the mobile station.
The xe2x80x98HAxe2x80x99 block in element 100 indicates that the SwMI also performs xe2x80x98Home Agentxe2x80x99 (HA) functions for radios which normally operate in the network. Finally, the xe2x80x98FAxe2x80x99 block in element 100 indicates that the SwMI also performs xe2x80x98Foreign Agentxe2x80x99 (FA) functions for radios which enter the network, but which normally do not operate in that network. The Home Agent and Foreign Agent functions will be explained in greater detail below and in connection with FIG. 4.
Mobile stations may be capable of operating in a number of different networks, for example those in different countries. This is referred to as xe2x80x98roamingxe2x80x99. Already, a radio designed according to the TETRA standard may roam from one network to another. Within the GSM and PCS 1800 digital mobile telephone standards, mobile telephones may roam between networks and/or countries.
In future, it will be desirable for mobile stations to be reachable easily through Internet Protocol (IP) addressing. It would be particularly desirable for IP addressable mobile stations to be able to roam from one network to another network.
Several schemes have been proposed to allow IP addressable mobile stations to roam within a cellular radio communications network, or between such networks.
Four examples of these schemes are explained below.
Mobile Internet Protocol
A fully mobile internet protocol system has been proposed. In this system, the mobile stations are capable of recognising when they are in a xe2x80x98foreignxe2x80x99 cell which is not part of the network within which they normally operate. The network within which they normally operate is the network in which they are under the control of their xe2x80x98Home Agentxe2x80x99. The home agent is provisioned with IP addresses for the mobile stations within the network within which the mobile station normally operates. A cell not under control of the home agent is referred to as being under the control of a xe2x80x98Foreign Agentxe2x80x99.
When a mobile station has moved from its home network to another, foreign network, data packets with IP addresses can still reach the mobile station. The mobile internet protocol allows the home agent to map and encapsulate the IP addresses of mobile stations into a xe2x80x98care ofxe2x80x99 address of the foreign agent, for forwarding to the foreign agent.
According to the mobile internet protocol proposal, the mobile stations will be able to transmit information to their Home Agent about the Foreign Agent where they are currently situated. This requires the mobile station to be adapted to recognise that it is not in a cell which is under the control of the Home Agent, and also to recognise the identity of the Foreign Agent, and to provide that information to its Home Agent as part of performing registration with the Home Agent.
iDEN Mobile IP radio
The IDEN Mobile IP radio has a mode where it can perform a mobile IP role as a proxy for non-mobile Data Terminal Equipment (DTE) attached to it. An example of such a DTE might be a lap-top portable computer. In this case, the iDEN radio must be arranged to perform the proxy function, and must itself be mobile IP capable in the sense described above.
Cellular Digital Packet Data Mobility
Cellular Digital Packet Data (CDPD) is a wireless data system used in the USA. In this system, IP addressing whilst roaming is possible only if both the infrastructure and the mobile station are adapted for the mobility function.
Thus in CDPD, IP address roaming is only possible for those handsets which have been adapted appropriately, and for these handsets, only within cells which can support roaming.
TETRA Packet Data Standard
The TETRA packet data standard EPT WG3(98) 005 describes the means for the SwMI to allocate an IP address to a Mobile radio. The IP allocation procedure is part of TETRA xe2x80x98context activationxe2x80x99. A mobile station performs context activation. Context activation is the act of a mobile station registering with a network to start sending and receiving packet data using IP addressing over the network. The network assigns an IP address to the mobile station at the time of context activation. Thus context activation allows the mobile station to gain an internet connection to send and receive xe2x80x98IP packetsxe2x80x99.
A TETRA radio does not need to perform context activation in order to simply conduct voice communication over a radio network. However, when the radio needs to send packet data over the TETRA network, then the radio must perform context activation. The TETRA radio may be commanded to perform context activation by the user. Alternatively, the radio may realise that it needs to perform context activation due to an outside stimulus, such as when the radio has been connected to a PC, and the PC boots up.
When context activation is complete, the network can map the IP address assigned to a radio to that radio""s Individual TETRA Subscriber Identity (ITSI). When the network receives a packet of data for the radio, then the network can locate the radio using the location map which it holds of the radios"" ITSI numbers. This then allows the network to forward the data packet to the radio""s location within the network.
The IP address allocated to a mobile station in a network can be either xe2x80x98staticxe2x80x99 or xe2x80x98dynamicxe2x80x99. A xe2x80x98staticxe2x80x99 IP address is normally an address which the mobile radio asks to use when it performs context activation. This will be the IP address which the mobile radio has used previously.
However, a radio may not specify any particular IP address at context activation. If this is the case, then there are two possible outcomes. Firstly, the network may recognise the mobile station, and allocate to the mobile station the same IP address as the network has previously assigned to that mobile station. This is static addressing, but without the mobile station having specified any IP address it its context activation request. The second possible outcome is that the network assigns any IP address to the mobile which is currently not in use, from the pool of IP addresses available to that network. This is xe2x80x98dynamicxe2x80x99 address allocation. The SwMI holds a pool of IP addresses for use in dynamic allocation. The pool of addresses available to one SWMI may differ from the pool available to another SWMI, for example that of a network in a different country, or that of a network owned by a different company.
The TETRA packet data standard allows the mobile radio to request a specific IP address to be approved by the SWMI. However, the current standard does not provide the means of roaming between different SWMIs.
Looking at the TETRA packet data standard in further detail, this standard defines 3 basic states of the Mobile radio. These are:
(i) Idlexe2x80x94This is the state prior to context activation.
(ii) Standbyxe2x80x94This is the state after context activation.
(iii) Readyxe2x80x94This is the state during data activity.
The mobile station has two timers. One of these is the xe2x80x98standbyxe2x80x99 timer. The standby timer measures the time since context activation. Typically the timer may be set to several hours from context activation before it xe2x80x98times outxe2x80x99, and the mobile station returns to idle mode. When the stand-by timer has expired, a mobile radio needs to perform context activation in order to once more commence data exchange over the network.
The other timer is the xe2x80x98readyxe2x80x99 timer. The mobile radio is in a ready state while it is in active operation, exchanging data packets over the network. The time limit of the ready timer is designed such that, when the ready timer has not yet timed out, the mobile station is likely to still be in an ongoing communication over the network. The ready timer times for typically 10 seconds after the most recent packet data activity. So the state of the ready timer specifies the state of the mobile radio, i.e. whether or not it is currently communicating. When the ready timer times out, the radio returns to the stand-by mode. Expiry of the ready timer does not cause the mobile radio to need to perform context activation in order to commence IP packet data transmission or reception.
In its home network, the mobile radio will keep one IP address until the stand-by timer times out. This means that the mobile will keep one IP address for a fairly long period after it has last sent or received a data packet using that IP address. This period might typically be 24 hours, during which the mobile radio is in stand-by mode. As long as less than this time has elapsed, the mobile radio does not need to perform context activation in order to commence IP data packet transmission or reception. If the mobile re-commences IP data packet transmission or reception when in stand-by mode, the mobile will re-set both the stand-by and ready timers.
However, a mobile radio also needs to perform context activation as soon as it enters a new network. Therefore a mobile TETRA radio will attempt to perform context activation when it enters a cell which is under the control of a Foreign Agent.
Individual TETRA Subscriber Identity Mobility
Individual TETRA Subscriber Identity (ITSI) mobility is a proposal for mobile station mobility within the TETRA standard. This mobility is not based on an IP standard. It requires resource from voice capacity sensitive elements of the infrastructure, such as the Home Location Register (HLR).
A need exists to provide enhancements to the systems and proposals of the prior art.